I met my current husband's children when their father and I had been seeing each other for about six weeks, and have now lived with them part-time for almost two years. I've been careful not to step on parental toes, perhaps a bit too careful; I think it's wise to go slow in establishing the stepparent/child relationship, but it's only very recently that I've really started to feel comfortable behaving like a "mom" with them, especially my stepson. I am happy that my stepdaughter has referred to me as her "second mom" on numerous occasions, including the day after I married her father, but I always want it to be clear I'm not "the mom." We're in a co-parenting relationship with her, and that needs to run smoothly; everyone needs to understand that I'm not trying to displace her in any way. Also, my son has a stepmother too now, and as "the mom" in that context, I know I don't want my role usurped.
There are experiences that are part of the family dynamic with multiple children--sibling rivalry in the forms of competition for stuff and attention, physical scuffles, screaming matches, being silly, blaming and teasing, sharing (willingly and under duress)--that just aren't present with an "only," and I'm learning how to manage these now as a second-time parent. My stepkids are great and I love them--bright, funny, usually well-behaved--but they're kids, they're opposite-sex siblings, and as my stepdaughter approaches her teens, the five years between them seems more like 20 sometimes. Traveling in the car is a lot noisier than it was with one child, for one thing, particularly in a Honda Civic with a fairly small back seat. Mediating disagreements is called for several times a day, and although it's strongly discouraged, tattling and blaming happen. (One thing about an only child is that it's always clear who did it, whatever "it" was.) There are turf wars sometimes over games, treats, and parental attention; since we're a part-time-custody household, that can be an especially tricky one around here, and the one I to which I try to be most sensitive. (I get their dad all to myself on the days the kids are with their mom, after all.) There's definitely twice as much patience required with two, and a need to teach and model it to them, along with sharing and fairness. But I think I'm at the right point in my life for handling the additional demands of multiple children, and I'm happy to have learned that, because once again, it's a life experience I wouldn't want to miss.